1. Field of the Invention
The present invention relates to the selective chlorination with gaseous chlorine of phenolic compounds which are substituted in the ortho positions with respect to the hydroxyl group.
2. Description of the Prior Art
Among the more important phenolic compounds which can be produced by chlorination of a phenolic compound which is substituted in the ortho positions, notable is 2,4,6-trichlorophenol.
The usual process for the preparation of 2,4,6-trichlorophenol consists of chlorinating 2,4-dichlorophenol.
However, a low proportion of 2,4,5-trichlorophenol is formed (on the order of 0.003 to 0.010% by weight of 2,4,6-trichlorophenol). 2,4,6-Trichlorophenol, which is an intermediate for the synthesis of other compounds, must contain only minimum trace amounts of this undesirable isomer.
Need therefore exists for processes enabling the chlorination of 2,6-dichlorophenol, which would totally avoid the formation of 2,4,5-trichlorophenol. 2,3,6-Trichlorophenol, which is the more likely to be formed in this case in trace amounts, is much less problematical than 2,4,5-trichlorophenol.
Indeed, when 2,6-dichlorophenol is chlorinated using gaseous chlorine, it is observed that an excellent yield is not obtained. In particular, a large amount of 2,4,5,6,6-pentachlorocyclo-2-hexenone is formed, which renders the reaction mixture very unstable and difficult to purify.
Cf. U.S. Pat. Nos. 4,160,114 and 4,223,166, FR-A-2,307,785, FR-A-2,584,068, Patent Abstracts of Japan, 11, No. 43 (C-402) [2490], Feb. 7, 1987, and JP-A-61/207,351.